1. FIELD OF THE INVENTION
The present invention relates to the determination of Poisson's ratio, particularly for thin layers.
2. DESCRIPTION OF THE PRIOR ART
It is known that the Poisson's ratio for a material can be determined from the ratio of the acoustic shear velocity to the longitudinal velocity. These velocities can often be easily measured using conventional ultrasonic time-of-flight techniques. Over the years, however, difficulties associated with making velocity measurements on thin attenuative specimens have forced ultrasonic researchers to become more innovative. For example, in an early effort to measure the velocity of both transverse and longitudinal waves in a buna-N vulcanizate as a function of temperature, A. W. Nolle and P. W. Sieck, J. Appl. Phys., Vol. 23, 888, (1952) employed a method involving the use of solid transmission media to conduct pulses into a thin flat specimen. The error associated with the acoustic measurement, however, was estimated in the report to be as high as 5% and 10% for the longitudinal and shear wave data, respectively. J. R. Cunningham and D. G. Ivey, J. Appl. Phys., Vol. 27,967 (1956) improved upon the aforementioned technique by incorporating a double acoustic path comprised of two separate specimens. Again, however, a number of experimental difficulties (e.g., producing uniform sample bondlines) contributed to significant measurement error. More recently, V. K. Kinra and V. Dayal, Experimental Mechanics, Vol. 28, 289 (1988) report combining standard FFT methods with conventional ultrasonics to measure the longitudinal phase velocity in specimens of submillimeter thicknesses. Their report includes a brief summary and comparison of their technique to those recently developed by others including the resonance method of F. H. Chang, J. C. Couchman and B. G. W. Yee, J. Comp. Mat., Vol. 8, 356 (1974) and the phase insensitive tone-burst spectroscopic method of J. S. Heyman J. Acoust. Soc. Amer., Vol. 64, 243 (1968) [5]. A primary object of the present invention is therefore to develop a schematic for making velocity measurements on thin attenuative specimen.